The present invention pertains to crystal filters for use in communications radios and the like, which filters have a number of poles depending upon the requirement of the radio. In the prior art, crystals having two resonators thereon are combined to form filters having a desired number of poles. In the design of a radio, for example, the sensitivity and selectivity are specified and the designer incorporates filters having the number of poles required to provide the specified selectivity and sensitivity. If, for example, a four pole filter will not provide the specified selectivity, the designer must go to a six pole filter, since he is restricted to combinations of two resonator crystals. However, increasing the filter by two poles, in many instances, (for example, when only minor increases in selectivity are required) may be relatively costly.
Typical examples of crystal filters utilizing two resonator crystals are disclosed in U.S. Pat. No. 3,633,134, issued Jan. 4, 1972 and entitled "Crystal Bandpass Filter Circuit"; U.S. Pat. No. 3,716,808, issued May 20, 1971, and entitled "Bandpass Filter Including Monolithic Crystal Elements With Resonating Portions Selected for Symmetrical Response"; and U.S. Pat. No. 3,947,784, issued Mar. 30, 1976, and entitled "Dual Coupled Monolithic Crystal Element For Modifying Response of Filter".